CK is dimeric molecule composed of M and B subunits, which are immunologically distinct. It exists as three main iso enzymes CK-MM, CK-MB, and CK-BB. The CK-MM is found in the muscle while CK - MB is found mainly in the myocardial cells. The CK - BB is found mainly in the brain and lungs, and enters the blood stream only on injury to these organs like cerebrovascular accident or pulmonary infarction. Normally CK - MB is found in the blood. CK - MB levels increase significantly 4 - 6 hours following a myocardial infarction and peak at around 12 to 24 hours after the infarct. The levels return to normal, in case of no further myocardial damage, after 24 to 48 hours. Hence the increased levels of CK - MB along with elevated levels of total CK is a good indicator of myocardial infarction. CK-MB levels usually do not rise in chest pain caused by angina, pulmonary embolism or congestive heart failure.
